This dress is magical. It's blue and white and, if you count the crown, red too. It's all made in Britain. It also makes me look at least 5ft 10in. Well, it and the stool I'm standing on make me look 5ft 10in. We had to, um, improvise, I'm afraid. It's shop stock - we couldn't start tampering with the hem. And even if we had, the proportions would have looked wrong (we gave the pins a tentative dry run. Boy do they leave an incriminating chain of tattoo marks on duchess satin).

This brings us neatly to the fundamental lesson of ball gowns. Master your sense of scale before it masters you and makes you look like Toulouse-Lautrec in drag. Ball gowns are all about being seen from a distance and having to live with the inevitable framed photographs for the next 50 years. If you are on the diminutive side (let us say under 5ft 5in, which is actually average, not that you'd know it from trouser lengths in this country), an empire line could be your greatest ally in the battle to maintain the illusion of long legs, especially once the inevitable hem shortening is completed.

Structure is key too. I thought this a little stiff when I first tried it, having envisioned myself in something floaty and romantic. What am I - six? Mentally, yes, because as I now realise, structure is the way forward after 40. Spindly straps are terrible unless you have spindly arms, and even then, they're debatable.

Truly there are many existential mysteries to get your head around in the ball gown matrix. Chief of which is why you are having to wear one.

Perhaps it is your company's centenary and the vice president of hole-punchers, not having that much to do in a paperless office, thought it would be fun to organise a grand gala in the town hall and invite Sir Tom Jones and Simon Cowell to judge the frocks; or maybe you work for Orange and you won Employee with the Smiliest Voice of the Week Award and now have to attend the Baftas; or you inadvertently became engaged to a Euro-royal and despite trying to hotfoot it out of the country now find yourself regularly hosting the Monégasque Bal de la Croix-Rouge .

Or maybe you got roped into organising what started as a simple, unpretentious little Jubilee celebration in a field, but somehow got out of hand and is doing for village relations what Gallipoli did for Anglo-Turkish solidarity. The point is, even those of us who make it a life ambition not to have to wear a ball dress usually end up having to wear one sooner or later.

My advice is thus: go with it, resistance is futile. So is trying to cobble something together at the last minute from your old maternity nightie, a pashmina and a mouldering curtain, even if it is Colefax and Fowler. Spontaneity may have worked for Scarlett O'Hara and Cinderella. They are fictional. Go for quality.

Consider renting. Girlmeetsdress.com is a good start - a smattering from Marchesa, Vionnet, Temperley and lots from Dinar Bar-El, an Israeli/LA designer I hadn't heard of but seems to do a line in glamour that falls just the right side of Dancing with the Stars. Or go vintage (atelier-mayer.com). Better fabrics, fewer zeros. Get it altered so it really fits. This will work miracles even on an inexpensive ball dress (generally the high street is hopeless on ball dresses but Topshop limited editions have been known to astound). Or splurge on a taffeta ankle-length skirt (cuter than full length) and wear it with a short-sleeve pastel cashmere knit, beaded top or shirt - very Carolina Herrera - and you'll get to wear that cashmere over and over again. Wear a short jacket, and moderate heel or a flat you can dance in. This is meant to be fun.

To celebrate the Jubilee, Harrods is producing a set of unique window displays in collaboration with these and other top designers and brands including Carolina Herrera, Valentino, Mulberry and De Beers; harrods.com